The present invention is directed towards methods for more efficient and economical separation of hydrocarbon gas constituents and recovery of both light gaseous hydrocarbons and the heavier hydrocarbon liquids. The present invention provides methods for achieving essentially complete separation and recovery of heavier hydrocarbon liquids. More particularly, the methods of present invention more efficiently and more economically separate ethane, propane, propylene and heavier hydrocarbons liquids from any hydrocarbon gas stream i.e. from natural gas or from gases from refinery or petroleum plants. Additionally, the present invention utilizes a process scheme which can be used for high ethane recovery during ethane recovery mode and high propane recovery during ethane rejection mode of plant operation. Thus, the process scheme proposed under the present invention provides additional flexibility to the plant operator to adjust to market conditions without requiring major process scheme changes.
In addition to methane, natural gas includes some heavier hydrocarbons with impurities e.g. carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas is further processed to separate and recover natural gas liquids (NGL). In fact, natural gas may include up to about fifty percent by volume of heavier hydrocarbons recovered as NGL. These heavier hydrocarbons must be separated from methane to be recovered as natural gas liquids. These valuable natural gas liquid comprises of ethane, propane, butane and other heavier hydrocarbons. In addition to these NGL components, other gases, including hydrogen, ethylene and propylene may be contained in gas streams obtained from refinery or from petrochemical plants.
Processes for separating hydrocarbon gas components are well known in the art. C. Collins, R. J. Chen, and D. G. Elliot have provided an excellent general overview of NGL recovery methods in a paper presented at Gas Tech LNG/LPG Conference 84. This paper, entitled xe2x80x9cTrends in NGL recovery for natural and associated gasesxe2x80x9d was published by Gas Tech, Ltd. of Rickmansworth, England, in the transactions of the conference on pages 287-303. The pre-purified natural gas is treated by well known methods including absorption, refrigerated absorption, adsorption and condensation at cryogenic temperatures down to xe2x88x92175 F. Separation of the lower hydrocarbons is achieved in one or more distillation towers. The columns are often referred to as de-methanizer or de-ethanizer columns. Processes employing a de-methanizer column separate methane and other volatile components from ethane and less volatile components in the purified natural gas liquids. The methane fraction is recovered as purified gas for pipeline delivery. The ethane and less volatile components, including propane, are recovered as natural gas liquid. In some applications, however, it is desirable to minimize the ethane content of the NGL. In those applications ethane and more volatile components are separated from propane and less. volatile components in a column generally called the de-ethanizer column.
Arn NGL, recovery plant design is highly dependent upon the operating pressure of the distillation column(s). At medium to low pressures i.e. 400 Psia or lower, the recompression horsepower requirement (to compress the residue gas to pipeline pressure) will be so high that the process becomes uneconomical. However, at higher pressures the recovery level of the hydrocarbons will be significantly reduced due to the less favorable separating conditions i.e. lower relative volatility inside the distillation column. Prior art has concentrated on operating the distillation columns at a higher pressure i.e. 400 Psia or higher while maintaining the high recovery of liquid hydrocarbons.
Many patents have been directed to methods for improving this separation technology. U.S. Pat. Nos. 4,596,588, 4,171,964, 4,278,457, 4,687,499, 4,851,020 describe relevant processes.
While prior art has been capable of recovering more than 98% of propane, propylene and heavier hydrocarbons during the ethane recovery mode, most of these processes fail to maintain the same recovery when ethane is unwanted i.e. in the ethane rejection mode. In order to achieve these goals for high propane recovery with ethane rejection, some systems have included two towers one operating at a higher pressure and one at a lower pressure.
A significant cost in the NGL recovery processes is related to the refrigeration required to chill the inlet gas. Refrigeration for these low temperature schemes is generally provided by using propane or ethane as refrigerants. Refrigeration is also provided by turboexpander processes as described below. For richer gases, containing a significant quantity of heavy hydrocarbons, a combined turbo-expander and external refrigeration process is the best approach.
In order to achieve the high propane recovery, with ethane rejection, some processes proposed in the prior art involve two tower processes i.e. these processes involve two distillation columns with one (de-methanizer) operating at a lower pressure than the other (de-ethanizer). Traditionally, for such processes four approaches to increase propane recovery have been proposed. The operating pressure of the de-ethanizer may be reduced. This approach often includes a two stage expander design to accommodate the high expansion ratio more efficiently. An alternative approach proposed in U.S. Pat. No. 4,251,249 is to install a separator downstream of the expander. This separator will separate the liquid and vapor from the partially condensed product from expander discharge. The vapor is combined with the residue gas from the de-methanizer and the liquid is sent to the de-ethanizer. However, the propane lost with the vapor from the separator reduces the recovery to 90% only. U.S. Pat. Nos. 4,657,571 and 4,690,702 propose processes which involve utilizing the overhead vapor from the de-ethanizer as a reflux in the packed top section of the de-methanizer. While high propane recovery of the order of 98% is achievable with this system, the increased recycle of methane and ethane increases the size of the de-ethanizer and both condenser and reboiler duties. In a related approach, U.S. Pat. No. 5,568,737 suggests recycling the residue gas stream from residue gas compressor discharge to be used as lean reflux in the upper portion of the de-methanizer.
In a typical cryogenic turbo expansion recovery process for high propane recovery with ethane rejection, a feed stream under pressure is cooled by heat exchange with other streams of the process and / or external sources of refrigeration such as propane compressionxe2x80x94refrigeration system. As the gas is cooled, liquid may be condensed and collected in one or more separators as high pressure liquid containing some of the desired C3+ components. Depending upon the richness of the gas and the amount of liquid formed, the high pressure liquid may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion causes a further cooling of the stream. Under certain circumstances pre-cooling the high pressure liquid prior to expansion may be desirable in order to further lower the temperature obtained after expansion. The expanded stream, comprising a mixture of liquid and vapor is fractionated in a distillation column (de-ethanizer). In the column, the expansion cooled stream is distilled to separate residual methane, ethane, nitrogen and other volatile components as overhead vapor product from the C3 components and heavier hydrocarbons obtained as bottom liquid product.
The vapor resulting from the partial condensation can be passed through a work expansion machine and expanded to a lower pressure. At this lower pressure further liquid will be generated from the gas due to partial condensation due to the cooling during expansion process. The expanded stream then enters a lower part of an absorber which operates at a pressure slightly lower than the pressure of operation of the de-ethanizer. In the absorber this expanded stream is contacted with the cold liquids to further knock out the C3 and heavier hydrocarbon components from the expanded stream. The resulting liquid stream obtained as a product from the bottom of the absorber is introduced into the upper section of the de-ethanizer column.
The overhead distillation stream from the de-ethanizer passes in heat exchange relation with the residue gas from the absorber column and is cooled, condensing at least a portion of the distillation stream from the de-ethanizer. The cooled distillation stream then enters the upper section of the absorber where cold liquids contained in the distillation stream contact with the expanded stream as described earlier. Typically the vapor portion of the distillation stream and the vapor overhead product from the absorber combine in a separation section in the upper portion of the absorber to yield a residual methane and ethane rich residue gas.
Prior art proposes to improve the propane recovery by utilizing the propane less lean overhead from the de-ethanizer as a reflux in the de-methanizer column. U.S. Pat. No. 5,771,712 recognizes that the absorber column and the de-ethanizer can be combined in a single column. In the proposed process the absorber is replaced by a reflux separator which separates a partially condensed stream obtained by cooling the side vapor draw obtained from the upper portion of the stripping section of the single column. However, in the proposed process the vapor from the reflux separator is combined with the methane and ethane rich residue gas obtained as the top product from the single column.
As can be seen from the foregoing description, prior art has long sought methods for improving the efficiency and economy of processes for separating and recovering propane and heavier natural gas liquids from natural gas. Accordingly, there has been a long felt but unfulfilled need for more efficient, more economical methods for performing separation.
The present invention is directed to processes for the separation of heavier hydrocarbons from a hydrocarbon containing gas under pressure. As mentioned above, prior art proposes several turboexpander processes, which aim at improving the heavy hydrocarbon recovery from a hydrocarbon mixture. Most of these processes involve operating two towersxe2x80x94a de-methanizer and a de-ethanizer.
Accordingly, it is the idea of the present invention to provide a process for separating components of a feed gas containing methane and heavier hydrocarbons, which maximizes the heavy hydrocarbon recovery from a distillation column, along with the ability to use the same process scheme for propane recovery (during ethane rejection mode) and ethane recovery modes of operation.
In carrying out these and other objects of the invention, there is provided, in the broadest sense, a process for cryogenically recovering components of hydrocarbon-containing feed gas in a distillation column, (e.g. a cryogenic distillation column) in which the top reflux (to the distillation column) is generated by processing the side draw obtained from the cryogenic distillation column. The lean reflux is defined as a reflux stream which contains very little of the component to be recovered from the cryogenic distillation column. The lean reflux can be generated by either partially vaporizing a side liquid draw from the cryogenic distillation column or by partially condensing a side vapor draw from the cryogenic distillation column and separating the streams so obtained to yield a lean vapor stream and a liquid stream. The lean vapor stream will then be completely or partially condensed by heat exchange with the residue gas and fed to the top of the cryogenic distillation column as a lean reflux. The liquid stream will be cooled, if required, and then fed to the top of the distillation column as a reflux. The two reflux streams obtained by such a process will have an enhanced separation effect as compared to the reflux used in prior art. This enhanced separation effect will enable the operator to obtain higher C2 or C3 recovery from the cryogenic distillation column by using said process.
The quality of the lean reflux can be further improved by replacing the separation step by a mass transfer step. In this process the side draw from the cryogenic distillation column will be fed to an absorber after cooling or heating the side draw, as appropriate. The absorber is a mass transfer device in which a portion of the side liquid or vapor draw from the distillation column or any similar hydrocarbon stream is contacted with another hydrocarbon stream to generate a lean vapor which can be condensed (partially or completely) by heat exchange with the residue gas from the distillation column to generate a lean reflux for the distillation column. The process involves introducing a partially or completely condensed hydrocarbon feed to the top of the absorber and contacting it with another stream which may be obtained byxe2x80x94withdrawing a vapor stream from the distillation column or by vaporizing a side liquid draw from the distillation column or by partially or completely vaporizing the liquid from the two-phase discharge obtained from the expander or by any such alternative means. The absorber comprises one or more mass transfer stages. A vapor stream, containing very little of component to be recovered, is generated from the top of the absorber. The feeds to the absorber and the conditions of operation of the absorber are so maintained that even though the lean vapor from the absorber contains very little of the component to be recovered but it can still be condensed (completely or partially) easily by heat exchange with the overhead residue gas from the distillation column. The predominant liquid stream obtained by substantially cooling and condensing the lean overhead vapor from the absorber is thereafter introduced to the top of the distillation column as lean reflux. The liquid stream from the bottom of the absorber is also introduced in the top of the distillation column as a reflux.
In one embodiment of the present invention the lean reflux process can be used for enhancing the heavy hydrocarbon recovery from a gas mixture containing methane, ethane, propane and heavier hydrocarbons. In this embodiment the feed gas is cooled and in some cases partially condensed. Any liquid condensed on cooling the gas is removed in a separator and is fed to the distillation column for further fractionation. The vapor from the separator is split into two streams. One stream is sent to the inlet of an expander and is expanded to the pressure of the distillation column. The partially condensed and cooled stream from expander discharge is fed to the middle of the distillation column. The second stream is substantially condensed by heat exchange with the overhead residue gas from the distillation column. This condensed stream is fed to the top of the absorber. A side vapor stream is withdrawn from the distillation column from a stage near the feed stage of the expander discharge stream. This side vapor draw stream is compressed and in some cases cooled to partially condense it and it is then fed to the bottom of the absorber, which contains one or more mass transfer stages. In the absorber, the cold liquid feed from the top knocks out the heavier components from the bottom vapor feed thus yielding a lean overhead. The liquid obtained from the bottom of the absorber is heavier and has a significant amount of ethane and propane and heavier components. This liquid is fed to the middle of the distillation column. The lean vapor from the top of the absorber is substantially cooled and condensed and is fed to the top of the distillation column as lean reflux. The conditions of the absorber and the feeds to the absorber are so maintained that the lean vapor from the absorber contains very little of the components to be recovered. Thus by maintaining these conditions, it will be possible to obtain a lean vapor which can be easily condensed by heat exchange with the top residue gas product and hence providing a lean reflux for the distillation column. Since the reflux stream generated from the lean vapor from the absorber overhead contains very little amount of the components to be recovered, the equilibrium loss (of the components to be recovered) in the residue gas from the upper portion of the distillation column is reduced to a minimum. Thus high recovery of heavier hydrocarbons can be achieved using the present invention.
In another embodiment of the present invention the top feed to the absorber can be obtained by partially or completely condensing the inlet feed gas in addition to or instead of partially or completely condensing the vapor from the expander inlet separator.
In another embodiment of the present invention, a side liquid stream, instead of vapor stream, is withdrawn from the distillation column and is pumped to an exchanger. In the exchanger this liquid stream is heated and in the process is partially or in some cases completely vaporized. The partially (and in some cases completely) vaporized stream forms the bottom feed to the absorber. In the absorber the partially or completely vaporized side liquid draw (from the distillation column) is contacted with a hydrocarbon stream which may be obtained by partially or completely condensing the inlet feed gas or by partially or completely condensing the vapor from the expander inlet separator. Due to the mass transfer effected in the absorber a lean overhead vapor is obtained. The lean vapor from the top of the absorber is substantially cooled and partially or completely condensed by heat exchange with the cold overhead residue gas from the distillation column. This (partially or completely) condensed stream is then fed to the top of the distillation column as lean reflux to achieve high recovery of heavy hydrocarbons.
An additional advantage of the process scheme covered under the present invention is that the same scheme can be used for high ethane recovery and high propane recovery with ethane rejection. Thus the same plant can be operated in an ethane recovery mode and an ethane rejection mode by making minor modifications to stream routing. Thus the process provides ability to obtain high recovery of desired hydrocarbons from a distillation column by using a side liquid or vapor draw from the distillation column as a bottom feed to the absorber which provides lean top reflux for the distillation column. This process can be used to obtain high ethane or propane recovery (with ethane rejection) with added flexibility to switch production targets in response to market conditions.